This invention relates to thermally imageable compositions and to stabilizers for these compositions.
It is well known that dyes in their reduced leuco form can provide the basis of color image forming systems. The leuco dyes may initially be relatively colorless, but can return to a colored form when oxidized, e.g., by air under acidic conditions or any other suitable oxidizing agent. Examples of leuco dyes used in color image forming systems include triarylmethanes, xanthenes, styryl dyes, and azine dyes, such as, for example, phenoxazines, phenothiazines, and phenazines.
In thermally sensitive materials of the type wherein at least one leuco dye is in reactive proximity with an inorganic nitrate salt, whereby imagewise application of heat causes said nitrate salt to oxidize said leuco dye to produce a change in color, a problem may arise from premature spotting or backgrounding of the thermally sensitive material during the drying step of the manufacturing process.
As used herein, the term "spotting" means oxidation of the leuco dye to a colored dye form in random irregular spots; the term "backgrounding" means oxidation of the leuco dye to a colored dye form in uniform fashion, resulting in an evenly colored background. Either spotting or backgrounding can destroy the usefulness of a transparency film bearing a thermally imageable composition. One method for preventing spotting involves drying of the imageable coating at low temperatures. This method, however, requires long drying times, slow coating speeds, and high costs, and in most cases, does not offer a practical solution to the problem.
In thermally imageable transparency films based upon combinations of leuco dyes and nitrate salts, it is essential that the thermally imageable compositions show considerable stability to the thermal effects of the manufacturing process in order to have a useful shelf life.
Isbrandt et al, U.S. Pat. No. 4,423,139 discloses a stabilizing agent for preventing oxidation and premature reaction of leuco dyes. The stabilizing agent comprises a combination of (1) one or more aromatic compounds which form quinones, diimines or quinonimines upon oxidation, or combinations of these, in combination with (2) 1-phenyl-3-pyrazolidinone (commonly known as phenidone), or derivatives of 1-phenyl-3-pyrazolidinone, or combinations of these. Although this stabilizing combination is good for preventing premature reaction of leuco dyes under storage condtions, it is not particularly useful for preventing premature spotting during the process of manufacturing thermally imageable material.